Variable resistor with improved terminal construction



1961 c. w. HARTMAN ETAL 3,012,215

VARIABLE RESISTOR WITH IMPROVED TERMINAL CONSTRUCTION Filed Nov. 23, 1959 2 SheetsSheet 1 dmdw pw EZznimn WHarLman 'S/izjlzam MFabmSn Dec. 5, 1961 c. w. HARTMA ETAL 3,012,215

VARIABLE RESIS w IMPROVED TERMINAL STRUCTION Filed Nov. 23, 1959 2 Sheets-Sheet 2 Jag I? United States This invention relates broadly to the seourement of the terminals of electrical control components, and partlcularly concerns the securement of the terminals of miniaturized variable resistors intended for use in elevated ambient temperatures, in which the resistance path is in the form of a metalized film chemically deposited upon one surface of a ceramic substrate or base.

Because of the extremely small size of these variable resistors-they are hardly more than one-half inch in diameter-and because of the inability of ceramic materials to withstand percussive forces to any appreciable extent, which makes the use of rivets or eyelets undesirablethe only practical way of securely fastening the terminals in place is to bond them to the base by means of a so-called glass to metal seal, bearing in mind the high operating temperatures-in excess of 175 C.-at which these resistors operate. To produce this seal, it is necessary to fire the stator assembly of the resistor, consisting of the ceramic base with the resistance path and its terminals in place thereon, at temperatures ranging from 500 to 750 C.

To avoid an undesirable change in resistance of the chemically deposited film forming the resistance path, during the firing, it is necessary that the firing of the stator assembly be conducted in an oxidizing atmosphere; but heating copper and other metals ordinarily used for terminals, unless suitably protected by a non-oxidizing plating, in an oxidizing atmosphere makes them unreceptive to solder. This constrains the choice of metal that might be used for the terminals to one which remains readily solderable, despite having been highly heated in an oxidizing atmosphere. Most of the noble metals and some of their alloys meet this requirement; but silver is the logical choice, because of its ready availability and relatively lower cost. The more expensive noble metals, and particularly gold, can be used where cost is not a serious factor and where greater resistance to oxidation is needed than that possessed by silver.

The relatively high firing temperature and the wellknown dielectric properties of steatite, makes it a first choice as the material to be used for the base, though other ceramics capable of withstanding high temperatures, as for instance alumina, could likewise be used.

This constrained choice of materials-steatite or alumina for the base and silver or gold for the terminalsposed what seemed to be an insurmountable problem in the securement of the terminals. As already noted, the ceramic base cannot withstand the impact shock involved in a riveted or clinched on securement. Hence, it followed that the only practical way of securing the terminals was to bond them to the base; but the great diiference in the linear coefficients of thermal expansion between steatite and silver or, for that matter, between alumina and gold, which might be used in their stead- 7x10" to 10 l()- perdegree centigrade for steatite and alumina, and 183x10 per degree centigrade and l4.2 10 per degree centigrade for silver and gold, respectively, would result in much larger stresses upon the seal or bond than known suitable sealing materials could withstand.

Still another factor had to be taken into account. The electronic component-variable resistor-with which this atent ice invention is primarily concerned, is intended for operation at elevated temperatures on the order of 275 C. Hence, the bonding material used must be inorganic. Ceramic or glass-like materials thus become a logical choice, but past experience indicated that these materials did not possess the required tensile strength to withstand the stresses involved. In addition, the usual method of glass sealing, wherein the metal terminal is surrounded by metal, can not be employed in this case. Here, the terminal passes through and is surrounded by ceramic, and this causes the terminals to pull away from the base during cooling from firing temperature, producing the indicated severe tensile stresses in the bonding material.

The object of this invention is therefore to provide a metal-glass-ceramic seal capable of withstanding very severe tensile stresses and thereby assuring permanently solid anchorage for the terminals of a variable resistor or other electrical control component designed to operate in high ambient temperatures in the neighborhood of 275 C., and in which the terminals are made from a noble metal and the base to which they are secured is formed of a ceramic material having a considerably lower linear coeificient of thermal expansion than has the noble metal.

The invention achieves this purpose through what is believed to be a novel structural relationship between the base of the resistor or other control device and its terminals, and through the discovery of a means for increasing the tensile strength of glass.

With the above and other objects in view which will appear as the description proceeds, this invention resides in the novel construction, combination and arrangement of parts and in the method involved in certain phases of the invention, all substantially as hereinafter described and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment of the hereindisclosed invention may be made as come within the scope of the claims.

The accompanying drawings illustrate one complete example of the physical embodiment of the invention constructed according to the best mode so =far devised for the practical application of the principles thereof, and in which:

FIGURE 1 is a view partially in side elevation and partially in longitudinal section through a variable resistor of the type with which this invention is particularly concerned, and illustrating one embodiment of the invention;

FIGURE 2 is a cross sectional View through FIGURE 1 on the plane of the line 2-2; and

FIGURE 3 is a. perspective View of the main elements of the stator assembly of the variable resistor shown in FIGURES l and 2, but with one of its resistance path terminals and the combined center contact and terminal shown separated from the base.

Referring now particularly to the accompanying drawings, the numeral 5 designates the base of a variable resistor made of ceramic material, such as steatite or alumina, for use in high ambient temperatures. The base has a resistance path 6 on its inner face and mounts terminals 7 and 8 for the ends of the resistance path, and a combined center contact and terminal 9. These are the major elements or parts of the stator assembly of the variable resistor.

The rest of the stator assembly comprises a cup-shaped housing 10 secured to the base over its front face or side, and a threaded mounting bushing 11 which projects coaxially from the housing and provides a bearing for the shaft 12 of the rotor assembly.

As is customary, the shaft 12 has a bridging contactor mounted on its inner end inside the housing. This contactor has a spring contact finger 13 bearing upon the center contact and another spring contact finger 14 bearing upon the resistance path through a contact shoe 15.

As in the variable resistor forming the subject matter of the copending Hartman et a1, application Serial No. 729,741, now PatentNo. (2,916,717, the resistance path 6 is a metalized film chemically deposited directly on the ceramic base, and preferably upon an annular plateau 16 which rises from the inner face of the base and is profitably glazed to assure the smoothest possible surface for the resistance path. At two circumferentially spaced points, the plateau 16 is interrupted to form notches or pockets 17 and '18 in which fingers 19 and 20 projecting laterally from the terminals 7 and 8, respectively, are received. These fingers 19 and 20 are electrically connected with the ends of the resistance path in a manner covered by the aforesaid copending application, and which involves silver soldering and the use of a gold bridging strip or band between the resistance path and the silver to assure against impairment of the electrical connection.

Being chemically deposited upon the glazed plateau 16,

Y which is an integral part of the ceramic base, the resistance path 6 is thus bonded directly to the base. The combined center contact and terminal 9, like the terminals 7 and 8., are stamped and formed of a noble metal, such as pure silveror gold, and allhave portions lying fiat against the inner face 21 of the ceramic base,

within its annular plateau -16.

For the terminals '7 and 8 the portions seated upon the face21 are the lateral extensions 19 and 20, and other shorter lateral extensions 22 and 23. By virtue of their shape and disposition with respect to the terminals 7 and 8. proper, the'lateral extensions 19, 20, 22 and 23 afford good stability for the terminals 7 and 8, and coact with the outer ends of the holes '24 and 25 in the base through which the terminals pass, to properly position the terminals on the base preparatory to their being anchored thereto.

For the combined, center contact and terminal, this positioning function and stability results from the fact that the center contact per se lies'fiat upon thesurface 21, together with the close fit between the terminal portion 9.in the outer end of the hole 26 in the basethrough which the terminal passes. A tongue 27 projecting from the center contact at a distance from its terminal 9' into a socket .28 formed in the base aids in the positioning of the center contact and terminal.

To enable .theouter end portions of the holes 24, 25 and26to effect their locating function they are of a size and shape to. snugly receive the terminal portions which pass therethrough, but from a point approximately midway in their axial length, the holes are taperingly enlarged to form sockets 24', 25, and 26-, which, like the socket 28 open to the inner face or side of the base. of these sockets, and for that matter any clearance between the smaller outer end portions of the holes and the terminal portions therein, are filled with glass seals 30.

The glass seals 30* are formed of a special glass which is characterized by exceptionally high tensile strength, so as to succasfully resist being cracked by the stresses imposed thereon by the tendency of the terminals to move with respect to the base due to the different linear coefficients of thermal expansions of the materials used in each.

To form a seal without cracks underthese circumstances, it is necessary to use a glass which will support large tensile stresses, and, if possible, will also reduce these stresses. A glass with a low softening point will minimize the stresses in the seal, since such glass is rigid over a relatively short temperature range. Lead borosilicate glass has a low softening point but when used to form the seals 30 was still prone to crack.

The art has recognized for some time that glass may be made more resistant to cracking by the incorporation therein of crystalline additives, which tend to distribute the stresses and also resist crack propagation. Likewise All the art has heretofore recognized that crystals. may be produced in a glass matrix by the addition of titanium dioxide to a glass batch in order to promote opacity in porcelain enamels.

But all attempts to produce a durable glass seal between materials suitable for use in the resistor of this invention, such as silver or gold for the terminals and steatite or alumina for the ceramic base, using crystal inclusions, as taught by the art, were unsuccessful.

Continued experimentation led to the discovery that when titania is added to lead boro-silicate glass, and the resulting'batch is smelted rapidly and then water-quenched to prevent crystal growth, a glass seal between the terminals and the ceramic base may be produced therefrom which is entirely satisfactory and'acceptable.

The proper amount of titania added to the batch to be used as a seal for a resistor having silver or gold terminals and a steatite or alumina base may range from about five to fifteen percent (5 to 15%). LikewiseJhe other ingredients to be used in such a batchfmay vary within the following limits:

- Percent Flint- 5 to 20 Boric acid 10 to 40 Litharge 50 to By way of example, one successfully produced batch of the above described glass comprised the following:

7 7 Percent Flint 7.0 Boric acid N... 29.3 Litharge c 53.7 Titania 10.0

After mixing, this batch was rapidly smelted and water-quenched. As a result, a glass frit (finely divided glass particles) was produced which had the following formula:

der thus prepared was then placed in the. sockets in. the

basearound the terminal portions therein, afterwhich the entire assembly-ie. only the stator assembly minus its housing and bushing-..'Was fired at a temperature of approximately 1200 F. for five minutes.

If desired, the powder need not remain in its dry state prior to being placed in the sockets, but instead may be mixed, with water to form a slurry, or mixed with a slight amount of water and used as a preform, to facilitateassembly of the components of the stator for firing.

Itisalso to be understood'that the temperature and period of time for firing the glass, as set forth above, is examplatory and merely constitutes one specific temperatureand time period which has been successfully employed. Other temperature and time periods may be used so long as the temperature for firing is not less than 500 C. (932 F.) nor more than 750 C. (1282 F,) and, the time period is. not less than three minutes-nor more than thirty minutes.

During the firing, the glass frit, of course, is fused and as it cools the titanium dioxide presumably crystallizes, thereby producing small crystals which are dispersed throughout the. body of the resulting glass seal and, by

seal enters. One such hole 31 is provided in the combined center contact and terminal directly at the bend which joins the contact and terminal portions.

While the foregoing description specifically recited sealing silver or gold terminals in a base of steatite or alumina, it is understood, of course, that other noble metals can likewise be used with other ceramic bases, by varying the composition of the sealing or bonding material depending upon the difference in the linear coefiicients of thermal expansion of the materials selected for use.

From the foregoing description taken in connection with the accompanying drawings, it will be apparent to those skilled in this art that this invention constitutes a valuable improvement in the electrical control component art, and in variable resistors designed to be used under extreme temperature conditions, as for instance those encountered in the control systems of rockets and guided missiles.

What is claimed as our invention is:

1. In an electrical resistor: a ceramic base for the resistor; means bonded to the base forming a resistance path; the base having a hole therethrough adjacent to one end of the resistance path; a metal terminal for the resistance path passing through the hole in the base and having a part thereof in juxtaposition to said end of the resistance path; means forming an electrical connection between said part of the terminal and the adjacent end of the resistance path; and means anchoring the terminal to the base comprising a body of glass filling the space in said hole not occupied by the terminal and fused to the base with the adjacent portion of the terminal embedded in the glass, said glass being a lead borosilicate glass containing between 5% and 18% titanium dioxide crystals to give it increased strength and good resistance to cracking to thus assure against shifting of the terminal with respect to the base and the resistance path thereon despite the relatively large difference in the linear coefiicients of thermal expansion of the ceramic base and the metal terminal.

2. The resistor of claim 1 further characterized by the fact that the hole in the base through which the terminal passes has a portion thereof of a size and shape to closely fit the terminal and thereby locate the same on the base, and another portion which opens to one side of the base and is substantially larger than the terminal portion therein so as to form a socket for the reception of the glass seal.

References Cited in the file of this patent UNITED STATES PATENTS 2,476,294 Hampton July 19, 1949 2,590,893 Sanborn Apr. 1, 1952 2,760,875 Schwartzwalder et al. Aug. 28, 1956 2,789,191 Arisman et al Apr. 16, 1957 2,829,062 Bennett et al. Apr. 1, 1958 2,839,414 Fenity et al June 17, 1958 2,863,782 Eubank et al. Dec. 9, 1958 2,882,173 Welsch Apr. 14, 1959 2,886,476 Dumesnil et al. May 12, 1959 2,898,395 Schurecht Aug. 4, 1959 

